1. Field of the Invention
The present invention relates to resin compositions which are useful as interlayer insulation materials for multi-layered printed wiring boards of the build-up type, in which insulation layers and conductive layers in which circuits are formed are alternately layered. The present invention also relates to adhesive films for multi-layered printed wiring board, which contain such said resin compositions. The present invention further relates to multi-layered printed wiring boards in which a cured product of the said resin composition is introduced into the insulation layer and also to methods for the manufacture of such multi-layered printed wiring boards.
2. Discussion of the Backgound
In recent years, as electronic devices such as portable phones are becoming smaller in size and higher in efficiency, and the build-up layer in multi-layered printed wiring boards is growing to many layers, there has been an increasing demand for a multi-layered printed wiring board having a multi-layered via structure called staggered via or stacked via where via holes are connected to plural build-up insulation layers.
Multi-layered printed wiring boards tend to be more and more multi-layered but, on the other hand, there is a need for achieving a thin form of multi-layered printed wiring board while their mechanical strength is to be still maintained. Prepregs and the like manufactured from glass cloth or the like, in which an epoxy resin is impregnated and which have been used in the conventional circuit substrate, have an excellent mechanical strength but there is a limit for reducing their thickness. There is a known method in which a multi-layered printed wiring board is manufactured using a copper foil to which a heat-resistant resin using a thermoplastic polyimide or the like is adhered. However, since the glass transition point of the heat-resistant resin is too high, the temperature necessary for lamination becomes high and its use for common multi-layered printed wiring board is difficult.
In the meantime, some of the present inventors reported, in International Laid-Open Patent Application No. WO 01/97582, that it is possible to form an insulation layer having an excellent mechanical strength by the use of an adhesive film having a heat-resistant resin layer and a thermosetting resin composition layer.
With regard to the heat-resistant resin used for circuit boards such as a multi-layered printed wiring board, commercially available films such as a polyimide film are generally used and, in order to form a conductive layer on the surface of such a film, it is necessary to carry out a roughening treatment by a dry method such as mechanical sanding or plasma etching. In International Laid-Open Patent Application No. WO 01/97582, the formation of thermosetting resin composition layers on both sides of the heat-resistant resin layer is adopted so that a roughening treatment by an oxidizing agent which is commonly used for the manufacture of multi-layered printed wiring boards by a build-up system and shows higher productivity is able to be applied. Thus, in an adhesive film for multi-layered printed wiring boards as such, the circuit substrate is laminated with a thermosetting resin composition layer on one side which is adjacent to the heat-resistant resin layer while another thermo-setting resin composition layer is roughened by an oxidizing agent whereupon a heat-resistant resin layer having an excellent mechanical strength is introduced together with lamination to the circuit substrate. Further, it is also possible that the thermosetting resin composition layer which is the outermost layer may be roughened by an oxidizing agent and then a conductive layer may be formed by means of metal plating whereupon a multi-layered printed wiring board having an excellent mechanical strength is easily manufactured. On the other hand however, when a three-layered structure, in which a heat-resistant resin layer is placed between the thermosetting resin composition layers, is adopted there is a problem that it is not possible to enhance the mechanical strength near the surface of insulation layer where the conductive layer is formed.
In International Laid-Open Patent Application No. WO 01/97582, as an example of layer constitutions of the adhesive film, there is further disclosed an example of the order of layers in which two layers of a heat-resistant resin layer and a thermosetting resin layer are the main layers. However, on page 17 of International Laid-Open Patent Application No. WO 01/97582, the above-mentioned three-layered structure for the roughening by an oxidizing agent is proposed as being mentioned that the surface of the insulation layer may be easily roughened and a conductive layer is easily formed by metal plating when a thermosetting resin composition layer (layer B) which is able to be roughened by an oxidizing agent is formed on the surface side of the insulation layer and, as a result, a direct roughening of the heat-resistant resin layer by an oxidizing agent is not expected. In addition, on pages 6 to 7, there are disclosures to the effect that heat-resistant resin varnish is applied and dried whereupon a heat-resistant resin layer is formed and that the said resin varnish may be used after kneading with appropriate thermosetting resin and resin additive therewith. However, there is no disclosure of any concrete examples at all.
In commercially available films such as polyimide film used as a heat-resistant resin layer in the Examples of International Laid-Open Patent Application No. WO 01/97582, it is not possible to avoid problems in terms of adhesion to a thermosetting composition layer having a different physical property. In addition, when the above-mentioned three-layered structure is adopted, it is not possible to enhance the mechanical strength of the area near the surface of the insulation layer where the conductive layer is formed. Further, there is also disclosed a film having a two-layered structure in which a heat-resistant resin layer and a thermosetting resin composition layer are adjacent and, when a commercially available film such as polyimide film is used as a heat-resistant resin layer, generally a roughening treatment by a dry method such as mechanical sanding or plasma etching is to be carried out in order to form a conductive layer on the surface thereof. Therefore it is desirable that a roughening treatment by an oxidizing agent which is commonly used in the manufacture of multi-layered printed wiring boards by a build-up system and is more highly productive is used.
In International Laid-Open Patent Application No. WO 01/97582, there is also a description that a heat-resistant resin varnish is applied and dried to make into a film as a heat-resistant resin layer and that a resin varnish is also used after kneading with an appropriate thermosetting resin and resin additive. However, there is only a general description and, with regard to compounding to a resin varnish, there is no disclosure of a specific constitution and there is also no disclosure of the matter of roughening at all. In conclusion, only the above-mentioned three-layered structure disclosed in International Laid-Open Patent Application No. WO 01/97582 is expected to be able to be roughened using an oxidizing agent.